Pressure sensors for use in applications such as tire pressure monitoring systems and the like need to be inexpensive, resistant to hostile environments, and consume very little power. This is especially important in battery powered systems because the sensor and related electronic circuits may operate for an extended period of time on a single coin sized battery.
By comparison, conventional pressure sensors which utilize piezo-resistive elements to measure changes in resistance to determine pressure are less power efficient which limits battery life. To overcome the problems associated with piezo-resistive elements, conventional capacitive type pressure sensors measure capacitance to determine pressure. One example of a capacitive pressure sensor utilizes MEMS technology wherein a diaphragm, capacitive electrodes, and a cavity are machined onto silicon. The capacitive sensor also includes electronic circuitry which measures the capacitance of the capacitive electrodes. To protect the diaphragm, the capacitive electrodes and the electronic circuitry from contamination in a hostile environment a cover is disposed over the cavity in the sensor package. However, because the diaphragm must be exposed to the pressure being measured, the cover must include one or more openings to allow exposure to the pressure being measured. The openings allow unwanted contaminants to enter the cavity which affects the capacitance being measured which leads to inaccurate measurements and can destroy the sensor.
Other prior art capacitive type pressure sensors, such as U.S. Pat. No. 5,436,795, incorporated by reference herein, overcome the problems associated with a cover having openings therein by employing a hermetically sealed cavity with two capacitive electrode plates sealed within the cavity to form a capacitive pressure measuring device. The design of the '795 patent incorporates a diaphragm into the cavity enclosure which deflects in response to the difference between the external pressure and the internal cavity pressure. One of the electrodes is located on the diaphragm and hence the capacitance is dependent on the pressure differential. However, the sensor as disclosed in the '795 patent has a limited spacing between the capacitive plates on the order of about 0.5 to 2.5 mils, and hence cannot accommodate any signal processing circuitry (e.g., an integrated circuit die). Hence, the design of the sensor as disclosed in the '795 patent requires any signal processing circuitry to be housed in a separate package which increases the size and complexity of the sensor.
Another prior art capacitive type pressure sensor is disclosed in U.S. Pat. No. 6,278,379, incorporated herein by reference. The sensor device as disclosed in the '379 patent includes a hermetic cavity with two capacitive electrodes disposed on the diaphragms on opposing sides of the cavity enclosure. The design provides for increased deflection of the capacitive electrodes which improves sensitivity. However, the '379 patent discloses that the gap must similarly be minimized to maximize the sensitivity of the sensor. In principle the cavity and gap could be enlarged to accommodate signal processing circuitry, e.g., an integrated circuit die; however, such a change would limit the minimum gap to greater than the die thickness (typically 600 um), and would also limit the area overlap between the capacitive electrodes, both of which would significantly reduce the sensitivity of the sensor to the applied pressure.
Other capacitive-based pressure sensors, such as Japanese Patent Application No. 2002039893, incorporated by reference herein, can provide a pressure sensor and a housing for electronic circuitry. This is achieved by employing a first cavity to accommodate electronic circuitry within the package and a second cavity, hermetically sealed, having two capacitive electrode plates sealed within the second cavity to form a capacitive type pressure measuring device. However, the design is complex, difficult to manufacture and expensive.